Multi-stage proportional control systems that are condition responsive are known. One of the major applications of this type of condition responsive control system is the control of heating and cooling equipment. The present invention is generally applicable to condition control systems that utilizes a condition responsive time proportional control, but will be described in terms of a thermostatically controlled system or thermostat.
In an electronic thermostat, anticipation can be achieved electronically. This has the advantage of not being affected by air flow and thus eliminates all of the problems associated with thermal anticipation. One method of obtaining this type of anticipation is the use of a resistor and capacitor charge and discharge arrangement as part of the negative feedback of an electronic amplifier while using a fixed positive feedback. This type of electronic anticipation is injected as a negative feedback mode with a single order time constant. For proper system operation, this time constant may need to be in the order of sixteen minutes. To obtain this type of a time constant with a single resistor-capacitor arrangement requires high resistances and a very low leakage, large capacitor. The size of the resistors and capacitor would place a burden on the cost of the device, and on the physical size of the thermostat itself, making electronic anticipation obtained in this fashion impractical for many thermostatic applications.
To obtain the desired time constant of approximately sixteen minutes, a relatively small capacitor and reasonably sized resistors can be used, thereby obtaining the relatively fast cycling rate in the time proportional control circuit. This relatively fast cycling rate can then be directly counted. If a counter is allowed to count up at a given rate during the "on" time of the anticipation, and another counter is allowed to count up at the same rate during the "off" time, we would have a digital representation of the "on" and "off" time periods for the desired operating condition (that is the actual deviation from the set point of the room temperature). The sum of these two counters is the cycling period. This type of information gives a complete description of the cycling pattern of the system for a constant input of a given magnitude. If the average room temperature and the set point remain constant, we would then let the cycling pattern continue but no longer allow the counters to count up. Each time the "on-off" action of the comparator or electronics occurs, the time counter would be reduced by a one count. When the counter reaches zero counts, the system will turn "off." The "off-on" action of the comparator or electronic amplifier would then start to count down the "off" time counter. When the "off" time counter reaches zero, the system would turn "on" and the counters would be allowed to count up at the given rate. This multiplies the "on" and "off" period by the number of counts stored in the counters. Since the basic "on" and "off" periods are determined by a constant, the concept also effectively multiplies by that same constant. To keep the system closer to the actual operating conditions, the "off" period counter can be updated each time the "on" period counter is counted down. Similarly, the "on" period counter can be updated each time the "off" period counter is counted down.
As thus described, the system will work well as long as the comparator is cycling. However, if a set point change is made or the deviation from the set point is such that the cycling stops, there is a possibility that the control can go out of "phase." That is, the furnace can be "on" when it should be "off," or the opposite can occur. Therefore, some means must be provided that will sense when these conditions occur and force the output into the proper state. One way would be to use two level detectors which could force the output into the proper state when the deviation from the set point is greater than the maximum anticipation signal or when the deviation is effectively negative. This method would involve a very critical calibration.
In the mentioned applications, a condition responsive time proportional control means has been specifically disclosed for a temperature responsive control means or a thermostat. The time proportional circuit utilizes a relatively small capacitor and resistors, and has a rapid cycling rate. This rapid cycling rate is sensed by a unidirectional counter that forms part of a counting means. The unidirectional counter, in one simple form, is a ripple counter. The cycling rate of the time proportional control means is combined with a pulse generating means so that the time constant of the overall control system can be multiplied by the pulse rate of the pulse generating means. The arrangement has the advantage in that the system can never go out of synchronization with the state of the condition being responded to even if there is a sudden change in the condition or a sudden change in the set point of the condition responsive system. The capacitor of the cycler or the cycling rate of the condition responsive time proportional control means can be changed to tune the cycle rate of the control system for any particular application without changing the system droop.